euh.. after checking my monitors.xml file, I ve seen that primary option was on left screen, I put it back to the right screen, reboot and finally the solution works, no more problem (till next time: I think that it is the application Mupen64 who is modifying this monitors.xml file when playing fullscreen)
–
BorisDec 31 '11 at 10:39

What is the resolution of both screens?
–
Thomas BoxleyDec 31 '11 at 11:58

1 Answer
1

After reading severals questions about the subject "multiple-monitors", I've found the solution.
In this answer I've discovered the existance of file ~/.config/monitors.xml. (thank you "pamar84")

So I did this:

do a backup of this file cp ~/.config/monitors.xml ~/.config/monitors.xml-bkp

open this file gedit ~/.config/monitors.xml

this file was containing 3 configurations !!!
So I did some cleaning:

I've found the configuration that I'm using and remove the 2 others.

also set my left screen to be the primary one.

Now with such nice ~/.config/monitors.xml file, Firefox is not disoriented anymore and have the good behavior:
- when my window is left side and I choose full-screen => full-screen is left screen
- when my window is right side and I choose full-screen => full-screen is right screen